LEDs have very low dynamic resistance, with the same voltage drop for widely varying currents. Consequently, they cannot connect directly to most power sources without causing self destruction. Therefore, there is a need for driver circuits.
LED drivers control the amount of current and voltage supplied to light emitting diodes. Well known in the art are LED driver circuits that employ analog electronic components. But analog electronic circuitry presents a plurality of inconveniences, among others, the need for a lot of auxiliary electronic circuitry (capacitors, resistors, etc.), their setting being possible only by way of changing the active or passive components, and in some cases their accuracy being strongly correlated with the accuracy of the electronic components.
Therefore, there is a need for controller based electronics that replace the currently present and widely used LED driver circuits on the market, that have only analog electronic components.
The LED illumination device may have a digital controller to control the LED output. The controller could be a pulse width modulator, pulse amplitude modulator, pulse displacement modulator, resistor ladder, current source, voltage source, voltage ladder, voltage controller or other power controller.
The light output of an LED is proportional to the forward current, so if the go forward current (IF) is not controlled properly, it can result in an unacceptable variation in light output. Also, exceeding the manufacturer's maximum IF specification can seriously reduce the LED's useful life.
In another approach power control is available via pulse width modulation that uses a fixed frequency of period T. The dimming is achieved by varying the pulse width. Multiple luminosity levels can be achieved for multiple duty cycles. Power control is achieved via Frequency modulation by using the concept of a fixed-width control pulse. Pulse A is always of the same duration. The luminosity is controlled by how often Pulse A repeats itself. Power control can further be achieved by Bit Angle Modulation that is based on a binary pulse train that contains the intensity value. Every bit in the pulse train is stretched proportionally to its significance. If the least significant bit b0 has a duration of 1, then bit b1 has a duration of 2, bits b2 through b7 have durations of 4, 8, 16, 32, 64 and 128 respectively.
While the various advantages that the use of microcontrollers presents in connection with managing the performance of an LED string, a plurality of limitations still present.
As far as voltages and currents, if VDD is the supply to both the LED and the microcontroller, then there is only enough voltage to drive one LED. Simple topologies do not allow for the LED voltage to be higher than VDD. For more LEDs in series such in a string, with the benefit of all being at the same current, VDD must be higher and requires a separate power supply for the microcontroller.
As far as the physical interfaces that support the communication, the microcontroller only provides simple synchronous (SPI) or asynchronous (SCI) communication. Additional hardware and software is needed to implement DALI, DMX, LIN and more. What is needed is a configurations that has less impact on cost, that is simple configuration for multiple LED's illumination apparatus.
As far as constant current regulation and switching speed, the key parameter in this application is switching speed. Larger inductors, which are more costly, are required for slower switching speeds. Most microcontrollers can accomplish an A/D conversion in about 15 μs. Add a few instructions to compare the read value to internal thresholds, and the conversion is up to 30 to 40 μs for the full analysis per ON or OFF cycle with an uncertainty of about 15 μs. This error dictates the minimum inductor value. Another approach is to set arbitrary ON and OFF durations, and then readjust these to try and accommodate the two current thresholds. This indirect method allows for a smaller, lower cost inductor, but it is less accurate. What are needed are accurate configurations for multiple LED's illumination apparatus.
As far as dimming and modulation speed, at 100 percent luminosity, there is no need to modulate the transistor. At the other extreme for the lowest luminosity level, i.e., one percent—it will be necessary to have the transistor on for one percent of the time. Given the fact that dimming must be done at 100 Hz or higher to avoid flickering, the PWM frequency must be 10 kHz or more. The eye can detect minute changes in the low luminosity range, and therefore 100 steps are not enough. If 4,000 steps were required (12-bit resolution), the PWM frequency would have to be around 400 kHz, which is almost impossible for a simple microcontroller.
Therefore the microcontroller based LED driver that would overcome the limitations discussed above has to at least provide solutions to the issued related to well-controlled, programmable, constant current source with high efficiency, processing speed and the impact on inductor size and dimming resolution, communication capability with industry standards, drive capability for multiple outputs and/or LED strings.
Several attempts have been made in the art to resolve the above referenced drawbacks and provide an optimal design for the microcontroller.
A proposed solution is illustrated by US 2007/0247305 A1 that teaches the use of a microcontroller for controlling LEDs by transmitting a ‘signature’ indicating the type of LED present. The optimum (nominal) current for the diode is then set using the transmitted signature information.
Therefore, based on all of the above, what is needed, specifically, is a system and method that remedies one or more of the drawbacks mentioned above in connection with the art.
What is needed is an electronic circuitry with power saving capabilities that at the same time allow for better thermal management. Further, what is needed is an algorithm to govern, control the LED power saving driving. At least two types of solutions are envisioned, a product and/or a measurement system providing an optimized driving and optimized thermal management.
Therefore the objective technical problem resolved by the present invention can be regarded at least as providing for an improved optimization apparatus and method for the total energy consumption of an illumination apparatus.
A further objective technical problem resolved by the present invention can be regarded at least as providing for said improved optimization apparatus and method for the total energy consumption of the LED string by calculating and optimizing a plurality of circuit parameters, for example the driving current, the modulation frequency, and the duty cycle of the PWM control.
These objective technical problems are solved by the present invention in accordance with the features of the independent claims. The dependent claims further develop the central concept of the invention in particularly advantageous ways.